Joe Bartlett
10/22/02
Geologic Analysis of the Beam
Abstract
The purpose of this report is to describe and interpret all of the geologic processes
visible at the limestone outcrop known as the Beam. The outcrop is located in South
Hero, Vermont and consists of shale with a distinctive layer of micrite limestone running
horizontally though it. This layer of micrite is referred to as the Beam. The Beam is
geologically significant for many reasons. The Beam contains evidence of multiple faultbend fold events which resulted in the shortening of the Beam. This deformation of the
micrite was calculated by measuring the amount of shortening due to both faulting and
folding. Detailed measurements of cleavage and fault planes and linear measurements of
Slickenlines were taken along all of the faulting surfaces. Observations of en echlelon
and sigmoidal veins lead to a fault formation model. The data taken at the Beam is
represented in several diagrams which display the sequence of shortening events,
cleavage deformation, stereonet plots of planar and linear data, and detailed diagrams of
vein structures. From the stereonet plots it is possible to determine the amount and
direction of compressional forces which acted upon the outcrop. The cleavage and fault
data is useful in determining the different responses of the shale and limestone to the
same compressional forces.
Introduction
The geologic formation known as the Beam is located in South Hero near the
intersection of McBride Lane and Route 2. The limestone outcrop is approximately 30
feet wide and consists of shale which is distinctly cut in half by a layer of lighter colored
micrite limestone. The purpose of this report is to describe and interpret the geologic
processes which formed the Beam as we observe it today. The Beam is very significant
to geologic study due to its representation of large-scale geologic processes in a small and
accessible location. The Beam contains multiple complete fault-bend fold mechanisms
including one duplex, which is two ramps stacked on top of each other. The Beam is also
very important because it demonstrates the different responses of two rock types to the
same compressional forces. The Beam contains all of the steps of ramp fault formation.
The progression of en echelon veins to sigmoidal veins to ramp fault is evident in many
places along the outcrop.
In order to interpret the geologic processes which formed the Beam, many
measurements were taken of faults, folds, and cleavage. A location map was created to
denote the exact location of the outcrop (Fig 1). The first step was to calculate the
amount of shortening of the Beam which was caused by faulting and folding (Table 1).
A diagram of all faulted and folded structures in the beam, the three types of cleavage,
and the vein structures was produced (Fig 2). We also observed cross cutting
relationships to determine the sequence of events and the connection with total shortening
of the Beam (Fig 3). Detailed cleavage measurements were taken in transects to study
the deformation of cleavage in the outcrop (Table 2). Stereonet plots were made to
represent the 3-dimensional planes we observed and were used to calculate the shear
strain which acted upon the outcrop (Figs 4+5). Finally, the shear strain was calculated
from the cleavage transect data (Figs 6+7, Table 4).
Data
Structure
Ramp 1
Ramp 2
Ramp 3
Ramp 4
Ramp 5
Non-ramp fold
total
Ramp
Shortening
(Distance
between
stadia rods in
inches)
12
7
2.5
15.5
20
Lo
(inches)
Lf
(inches)
Fold
Shortening
(Lo-Lf)
Total
Shortenin
g
(inches)
Percent
Shortening
(Ts/original
length)
29
33
27
58
18
36
25
29
26
45
14
33
4
4
1
13
4
3
16
11
3.5
28.5
24
3
86
19.4%
Table 1: Shortening of the Beam data as measured using stadia rods and a tape measure. The original length was found to be 37 feet
or 443.5 inches.
Location of transect
Under Ramp 1
Under Ramp 2
Under Ramps 3 + 4
Under Ramp 5
Type
S1
Sr
Sr
Sr
Sf
S1
Sr
Sr
Sf
S1
S1
Sr
Sf
S1
S1
Sr
Sr
Sf
Strike
050
054
050
060
024
044
041
036
065
021
018
018
025
050
044
066
039
032
Dip
56 SE
32 SE
24 SE
18 SE
10 SE
55 SE
38 SE
32 SE
21 SE
67 SE
61 SE
38 SE
22 SE
63 SE
62 SE
52 SE
32 SE
22 SE
Table 2: Data from measurements of cleavage planes along 4 transects. Transects ran from S1 cleavage to the Sf cleavage below the
beam near each areas of ramp faulting.
Fault
Ramp 1
Ramp 2
Ramp 3
Ramp 4
Ramp 5
Floor Fault
S.L. Trend
347
328
358
357
342
335
330
335
332
334
Plunge
28 SE
24 SE
42 SE
31 SE
12 E
08 NW
09 NW
09 NW
11 NW
11 NW
Table 3: Fault plane data as taken from Slickenlines along each Ramp fault and several locations along the Floor Fault.
Location of cleavage transect
West of Ramp 1
Under Ramp 2
Under Ramp 3+4
Under Ramp 5
Angular shear (degrees)
50
36
44
40
Simple Strain
1.19
0.73
0.97
0.84
Table 4: Shear strain results as calculated from the tangent of angular shear which was determined from the cleavage transect
stereonet plots (Figure 4)
Results
The Beam consists of a distinct layer of micrite limestone located in a shale
outcrop. The shale is observed to be heavily cleaved whereas no cleavage is found
cutting through the beam. The micrite limestone differs from the surrounding shale in its
composition. Shale is a form of limestone with large percentage of interstitial calcite
cement binding the silt-sized particles. Micrite limestone is more densely packed and
consists of larger grain sizes tightly packed with microcrystalline calcite. A distinct
boundary is visible between the shale and the lower contact with the Beam. Directly
above the beam, patches of rock are visible which appear to be more affected by
fracturing than cleaving (Fig 2).
Faults
A fault can be characterized as showing signs of movement: Slickenlines,
truncations, and deposits of calcite. The Beam contains a floor fault, ramp faults, and
roof faults. The floor fault is observed to mark the lower the boundary of the Beam
within the outcrop. Five distinct ramp faults are visible cutting across the beam.
Multiple roof faults are visible above the Beam and appear to be concentrated near the
areas containing ramp faults. Slickenlines, which are parallel grooves found in calcite
deposits, are found throughout all three varieties of faults present in the Beam. Large
calcite deposits are found along the floor fault with lesser quantities of calcite present
within the ramp and roof faults (Fig 2). A different looking ramp fault zone was visible
near the center of the outcrop. This zone is known as a duplex, where two separate ramps
are stacked on top of each other. Ramp 3 is located closely to the east of Ramp 4 and
appears to be much steeper than the other ramps. A larger amount of fracturing is also
visible in the material above the ramps (Fig 2).
Cross-cutting relationships of the faults are needed in order to determine the
relative age of the events represented in the Beam. Several cross-cutting relationships are
observed along the ramp and roof faults. Each of the five ramp faults truncate the roof
fault located to the East. The ramp faults are observed to continue into the roof fault
located to the West of the ramp. This pattern continues across the outcrop (Figs 2 + 3).
The faulting is observed to shorten the overall length of the Beam by stacking
layers of micrite on top of each other along each ramp fault. This lifting and stacking
shortens the overall length present before faulting. In order to calculate the overall
shortening of the beam it is necessary to locate the exact point of truncation both along
the ramp fault and roof fault of each of the five ramps (Fig 2). A thin layer of distinctive
material is present near the top of the Beam. This layer is useful in locating the
truncations necessary to calculate the amount of shortening due to the ramp fault. Stadia
rods were placed on each of these truncations, or “T” junctions, and aligned
perpendicular to the plane of the fault, as determined through the trend and plunge of
Slickenlines. The distance was then measured between the poles. A tape measure was
used to calculate the amount of shortening due to the folding associated with each fault,
and with the non-ramp fold. The tape was stretched along a bedding plane through the
fault to calculate Lo and the direct distance from one side of the fault to other was
measured as Lf. The current length of the Beam was determined through a triangulation
measurement to be 29.8 feet. The shortening due to faulting and folding along the Beam
was then calculated by adding up all of the shortening events to calculate the initial
length of the Beam, and then dividing the current length by the original length (Table 1).
The structure of the ramp faults are fairly consistent across the outcrop. The
structure of the ramp faults is determined through measuring the plane formed by each
ramp. The Slickenlines are also measured on each ramp. This data is plotted on equal
area stereonets (Fig 5). Both the fault planes and the Slickenlines trended to the North
East.
The veins structures present in the Beam appear to be similar to the geologic
structures known as en echelon and sigmoidal veins. These veins which are somewhat
ellipsoidal in shape, are found in several locations along the Beam. Other veins are more
“s” shaped and some of these are observed lining up in an angled line going across the
Beam. Most of the faults have shark-toothed veins present which looked similar to those
near the sigmoidal veins (Fig 2).
Cleavage
The structure and density of cleavage is a major feature of the outcrop. Spaced
cleavage dominates the shale beds both above and below the Beam. No cleavage is
visible cutting through the Beam. The spaced cleavage (S1) has a fairly consistent dip
and spacing across the outcrop. Closer to the Beam, the cleavage appears to bend to a
more shallow dip (Fig 6). Across most of the outcrop the bent cleavage, or rotated
cleavage (Sr) ends abruptly into the fault zone cleavage (Sf) present under the Beam
(Table 2). In a few areas it is visible to see the rotated cleavage continue in the same dip
into the fault zone cleavage (Fig 2). The fault zone cleavage has variable dips across the
outcrop with the most horizontal visible on the eastern side. The fault zone cleavage
consists of loosely packed tightly bedded clay and fine silt sized particles. The fault zone
cleavage appears to be thicker near the ramp faults. Similar patterns of spaced cleavage
transitioning into rotated, then fault zone cleavage is also visible above the micrite layer.
The rotation above the micrite layer appears to be in the opposite direction of that found
below the Beam (Fig 2).
Discussion
Sequential Evolution of Outcrop
The sequential evolution of the limestone outcrop is determined through crosscutting relationships. Each ramp fault is observed to begin out of the floor fault and then
truncate the roof fault to the east. This implies that the roof fault was already present
before the formation of the ramp fault. This progression continues across the outcrop
from east to west. Using this model, the eastern most ramp: Ramp 1, formed first and the
western most ramp: Ramp 5, form most recently. The floor fault formed progressively
to the west with each ramp faulting event. The floor fault is continuous due to the
confinement of faulting to the narrow bed of micrite limestone (Fig 3). This progression
links the structures in a series which is supported by the fault-bend fold model. The faultbend fold model shows the development of a fault which transistions from a floor to ramp
to roof fault. In the Beam this roof fault is then truncated by the next ramp fault showing
that the second ramp must have formed more recently.
The duplex formation is somewhat different from the rest of the ramp fault
progression. In the duplex, the eastern fault, ramp 3 formed first, lifting and folding
some of the beam onto itself. After this ramp stopped moving, a new ramp, ramp 4
formed to the west and pushed up more material (Fig 3). This caused the deformation of
ramp 3 and explains the drastically steeper dip of this fault plane.
The measurements of slickenlines taken along the Floor fault suggest that the
compressional forces were fairly consistent (Table 3 + Fig 5). The floor fault was
forming throughout the ramp faulting events and the strong similarity between the data
suggests that the direction of force was very similar throughout the process. The varying
degrees of shortening associated with each ramp fault implies that the amount of force
was not constant during formation. The ramps where large amounts of material were
folded would have occurred under higher pressure (Fig 3).
Deformation of Cleavage
The deformation of cleavage can also be used to interpret the geologic forces that
formed the outcrop. The cleavage was formed as a response of the shale to the same
compressional forces which fractured the beam. When compression is applied to shale
which has much higher amounts of calcite and water than micrite, a pressure solution is
formed. This pressure solution takes the form of cleavage. The cleavage forms where
calcite is dissolved out of the rock leaving behind fine sized particles. This cleavage
must have formed before the faulting because it is deformed by the movement of faults
along the Beam. This deformation is evident through all four of the cleavage transects.
Each transect shows that with increased proximity to the Beam, deformation increases
(Fig 6). The amount of deformation between lower spaced cleavage and fault zone
cleavage suggests that the shear strain was fairly constant throughout the deformation of
the Beam (Fig 7).
The degree of deformation of the cleavage can be used to support the fault
sequence. The cleavage transects show progressively more deformation moving to the
east (Fig 4). This is due to the continuous movement of the floor fault to the east of each
new ramp fault. This explains the more horizontal dip of fault zone cleavage found near
Ramp 1. The wave of cleavage formation also supports the faulting model because the
cleavage must have formed before the faults in order to provide the lubrication needed for
the rocks to move (Fig 3). The faulting of the Beam required water to lubricate the
movement. This is evident through the large deposits of calcite present throughout the
faults (Fig 2). The thickness of calcite under the Beam shows that forces were pushing
the water upwards through the shale where it collected below the relatively impermeable
Beam. This water was already present when the Beam began to fracture.
Response of Outcrop to Compressional Forces
The sequential formation of cleavage and faulting demonstrate the different
reactions of micrite and shale limestones to the same compressional forces. When the
compressional force was first applied to the outcrop, the shale which is much weaker
reacted by forming a pressure solution while the beam built up stress until enough stress
and enough water pressure were present for the ramp fault to form through a series of
sigmoidal veins. Once the ramp developed, the beam was compressed and folded in on
itself to relieve the pressure. The beam continued to be pushed up the ramp fault until the
compressional force was exceeded by the amount of force needed to push more material
up the ramp. Once this happened, more cleavage began forming to the west accompanied
by another ramp fault. This process continued across the outcrop (Fig 3). The shortening
of the Beam was also directly linked to the amount of material which had to have been
removed from the shale beds. The Beam was shortened by 19.4%, this implies that the
shale beds lost approximately the same amount of material to pressure solution. This
large amount of material loss explains the high density of S1, Sr, and Sf cleavage visible
in the shale (Fig 2).
Conclusions
The Beam represents a very rare geologic opportunity to see many major
processes in their entirety. The outcrop contains five complete fault-bend fold events
including a duplex. These events occurred in a progression which was determined to go
from east to west through cross-cutting relationships and cleavage deformation. The
cleavage deformation present shows the rotation caused by the shear strain from the
movement along the Floor Fault. The outcrop also contained evidence of the progression
of en echelon vein formation leading to ramp fault development. These veins represent
the response of the densely packed micrite layer to the same compressional forces which
caused the extensive development of cleavage in the shale. The geologic processes
visible in the outcrop and the interpretations which can be drawn from them demonstrate
the importance of the Beam.